This invention relates to a low temperature aluminum reduction cell and more particularly, it relates to a low temperature aluminum reduction cell using an improved method for removing molten aluminum from the cell.
The present invention relates generally to methods and apparatuses for the electrolytic reduction of alumina to aluminum. More particularly, the subject matter herein relates to the subject matter disclosed in Beck et al. U.S. Pat. No. 4,592,812; 4,865,701; 5,006,209; 5,284,562; and U.S. patent application Ser. No. 09/247,196, the disclosures thereof which are incorporated herein by reference.
The aforementioned patents of Beck et al. are directed to a series of developments relating to the electrolytic reduction of alumina to aluminum. The developments culminated in an electrolytic reduction cell containing a relatively low melting point, molten electrolyte composed of fluorides, a non-consumable anode composed of a particular alloy of copper, nickel and iron, and a cathode, composed of titanium diboride (TiB2), that is wettable by molten aluminum. A plurality of the non-consumable anodes are vertically disposed within a vessel containing a bath composed of molten electrolyte. A plurality of the cathodes are also vertically disposed within the vessel, with the cathodes being arranged in close, alternating, spaced relation with the vertically disposed anodes. In a preferred embodiment, the vessel has an interior metal lining electrically connected to the anodes and having essentially the same composition as the anodes. The lining can function as an auxiliary anode.
The bath of molten electrolyte contains dissolved alumina and additional alumina in the form of finely divided particles. The molten electrolyte has a density less than the density of molten aluminum and less than the density of alumina. As noted above, some alumina is dissolved in the molten electrolyte. When an electric current is passed through the bath, aluminum ions are attracted to the cathodes, and oxygen ions are attracted to the anodes. Bubbles of gaseous oxygen form at each of the anodes, and aluminum forms at each of the cathodes. The bubbles of gaseous oxygen pass upwardly from the anodes and maintain the undissolved, finely divided alumina particles suspended in the bath of molten electrolyte, forming a slurry. The metallic aluminum formed at the cathodes wets the surface of each cathode and flows downwardly along the cathode.
The electrolytic reduction cell is operated at a relatively low temperature, substantially below 950xc2x0 C. The composition of the electrolyte employed in the cell enables operation of the cell at a relatively low temperature, because the electrolyte is molten at that low temperature. The low cell temperature allows the use of non-consumable anodes composed of the Nixe2x80x94Cuxe2x80x94Fe alloys described below without subjecting the anodes to deterioration in the molten electrolyte.
In conventional electrolytic cells for production of aluminum from alumina, molten aluminum is removed by tapping the cell periodically by removal of a plug in the bottom of the cell where the molten aluminum has collected. In such cells, the density of the molten aluminum is greater than the density of the electrolyte. Consequently, molten aluminum collects on the floor of the cell which may be comprised of the cathode. In another embodiment, the aluminum is removed by siphoning molten aluminum from the pool of metal collected on the cell floor. However, when the aluminum is not permitted to collect on the floor of the cell, then its removal becomes much more difficult and thus various processes have been proposed.
U.S. patent application Ser. No. 09/247,196 discloses capillary action for collecting metal product. One problem with capillary action is clogging of the capillary conduit which can result from freezing of the electrolyte when a shift in current density occurs. Another problem resides in removing metal from the capillaries because the molten metal is drawn towards the smallest cross section. In addition, there is the problem of excluding alumina particles from the product metal when an alumina slurry electrolyte is used with capillaries.
U.S. Pat. No. 5,284,562 discloses an oxidation resistant, non-consumable anode, for use in the electrolytic reduction of alumina to aluminum, that has a composition comprising copper, nickel and iron. The anode is part of an electrolytic reduction cell comprising a vessel having an interior lined with metal which has the same composition as the anode. The electrolyte is preferably composed of a eutectic of AlF3 and either (a) NaF or (b) primarily NaF with some of the NaF replaced by an equivalent molar amount of KF or KF and LiF. In this patent, one embodiment of a removal device is a pierced, titanium diboride member 31 which is wet internally and externally by aluminum and is mounted in the lower, inlet end of a suction tube 32 disposed above tap location 34. Member 31 has a lower-most extremity at tap location 34. A sump (not shown) may be provided at tap location 34 to assist in accumulating molten aluminum there. Titanium diboride member 31 will remove molten aluminum from the cell.
U.S. Pat. No. 4,740,279 relates to a process of producing lithium metal by the electrolysis of fused mixed salts comprising electrolyzing fused mixed salts consisting of lithium chloride and potassium chloride in a diaphragmless electrolytic cell, withdrawing molten lithium metal from the cell to a receiver and cooling the lithium metal which has been withdrawn. To decrease the content of impurities in a continuous process, molten mixture which rises in the interelectrode space in the cell and contains lithium metal is collected in an annular zone, which surrounds the top end of the cathode adjacent to the surface level of the molten mixture, the molten mixture is withdrawn from the annular zone through a siphon pipe and is supplied from the latter to a separating chamber, which communicates with the electrolytic cell and is sealed from the chlorine gas atmosphere in the electrolytic cell. Electrolyte and lithium are separated in the separating chamber under a protective gas atmosphere. Lithium metal is discharged from the separating chamber into a receiver under a protective gas atmosphere and the electrolyte is recycled from the separating chamber to the electrolytic cell.
U.S. Pat. No. 4,165,272 discloses an electrolytic cell cathode having a hollow cathode finger with fins extending outwardly therefrom for electrolysis of alkali metal chlorides. A synthetic separator surrounds the cathode and rests upon the fin-like extensions.
U.S. Pat. No. 4,681,671 discloses a method of producing aluminum by electrolysis of alumina dissolved in molten cryolite at temperatures between 680xc2x0-690xc2x0 C.
The method comprises the employment of permanent anodes the total surface of which is increased up to 5 times compared to the total surface of anodes in a classical Hall-Heroult cell of comparable production rate. By this means the anodic current density is lowered to a degree which permits the discharge of oxide ions preferentially to fluoride ions at an acceptable rate. Additionally, the electrolyte is circulated by suitable means whereby it passes from an enrichment zone where it is saturated with alumina to an electrolysis zone and back.
U.S. Pat. No. 5,498,320 discloses a method and apparatus for electrolytic reduction of alumina using a porous cathode. The patent discloses in aluminum smelting by electrolysis, a double salt of KAlSO4, as a feedstock, heated with a eutectic electrolyte, such as K2SO4, at 800xc2x0 C. for twenty minutes to produce an out-gas of SO3 and a liquid electrolyte of K2SO4 with fine-particles of Al2O3 in suspension having a mean size of six to eight microns. This is pumped into a cell with an electrolyte comprised of K2SO4 with fine-particles of Al2O3 in suspension, an anode and a porous cathode of open-cell ceramic foam material. The cell is maintained at 750xc2x0 C. and four volts of electricity applied between the anode and the cathode causes oxygen to bubble at the anode and liquid aluminum to form in the porous cathode. A channel within the porous cathode, and the porous cathode itself, are deep enough within the cell electrolyte that the pressure head of electrolyte is enough to overcome the difference in density between the molten aluminum and the electrolyte to pump molten aluminum from the channel out of the side of the cell. The electrolyte K2SO4 is periodically bled-off to control a build-up of the material as aluminum is produced form the double salt of K2SO4.
U.S. Pat. No. 5,855,757 discloses a process for the production of a molten metal by electrolysis in an electrolytic cell having an electrolysis compartment, a metal recovery compartment, and a partition separating upper parts of the compartments, the process comprising: electrolysing in the electrolysis compartment an electrolyte containing a fused salt of the metal the electrolyte being of greater density than the metal; continuously withdrawing the product metal mixed with the electrolyte in a stream from the electrolysis compartment to a top part of the metal recovery compartment; allowing the metal to form in the metal recovery compartment as a pad floating on the electrolyte; maintaining the pad out of contact with the partition; and recovering the pad.
U.S. Pat. No. 5,368,702 discloses a multimonopolar cell for electrowinning aluminum by the electrolysis of alumina dissolved in a molten salt electrolyte, comprises electrode assemblies each having a non-consumable anode and a non-consumable cathode both resistant to attack by the electrolyte and by the respective product of electrolysis. The anode (2) is preferably of tubular form with an active anode surface (7) inside, and the cathode is made of one or more rods (1) or tubes placed in the middle of the tubular anode or between plate anodes, the cathode extending beyond the bottom of the anode.
U.S. Pat. No. 5,415,742 discloses a process for electrowinning metal in a low temperature melt. The process utilizes an inert anode for the production of metal such as aluminum using low surface area anodes at high current densities.
It is an object of this invention to provide an improved electrolytic smelting cell for producing aluminum from alumina.
It is another object of this invention to provide an improved means for collecting molten aluminum produced from alumina in a low temperature electrolytic cell.
It is still another object of the invention to provide a cathode collection means for collecting molten aluminum produced from alumina in a low temperature (less than 900xc2x0 C.) electrolytic cell employing an inert anode.
Yet, it is still another object of this invention to provide a hollow cathode for collecting molten aluminum therein produced from alumina in an electrolytic cell employing an inert anode.
And yet, it is still another object of the invention to provide an improved process for producing aluminum from alumina in a low temperature electrolytic cell employing an inert anode and an electrolyte containing alumina in an amount greater than solubility and a hollow cathode for collecting molten aluminum therein.
These and other objects will become apparent from a reading of the specification and claims appended hereto.
In accordance with these objects, there is provided a method of producing aluminum in an electrolytic cell containing alumina dissolved in an electrolyte, the method comprising the steps of providing a molten salt electrolyte having alumina dissolved therein in an electrolytic cell having a liner for containing the electrolyte, the liner having a bottom and walls, the liner being substantially inert with respect to the molten electrolyte. A plurality of non-consumable anodes are disposed substantially vertically in the electrolyte along with a plurality of hollow cathodes. Each cathode has a top and bottom. The cathodes are disposed vertically in the electrolyte and the anodes and the cathodes are arranged in alternating relationship. The anodes have a substantially plate-shaped configuration. Each of the cathodes is comprised of a first side facing a first opposing anode and a second side facing a second opposing anode. The first and second sides are joined by ends to form a reservoir in the hollow cathode, the cathode having a bottom opening and a top opening into the reservoir. An electric current is passed from the anodes, through the electrolyte to the cathodes, depositing aluminum on the cathodes and oxygen bubbles are generated at the anodes, the bubbles stirring the electrolyte. Molten aluminum is deposited at the cathodes and collected in the reservoir in the hollow cathodes. A portion of the molten aluminum is withdrawn from the reservoir through the top opening.